Vehicles may include a catalyst for processing engine emissions. Performance of a catalyst may degrade over time due to the catalyst being exposed to elevated temperatures, mechanical vibration, sulfur contamination, and exposure to other operating conditions that may be encountered during a course of operation. If a catalyst degrades by more than a desired amount, vehicle tailpipe emissions may be elevated to an undesirable level. Thus, it may be desirable to provide a robust way of judging if a catalyst is performing as desired.
One way of diagnosing operation of a catalyst is to use oxygen sensor information. In particular, oxygen sensors may be placed in an exhaust system upstream and downstream of a catalyst. Information from the upstream and downstream oxygen sensors may be a basis for judging whether or not a catalyst is performing in a desired way. However, oxygen sensor performance may degrade over time. Output of a degraded oxygen sensor may lead to judging that a poor performing catalyst is performing in a desired manner. Therefore, it may be desirable to provide a way of separating oxygen sensor performance from catalyst performance when judging if a catalyst is operating as desired.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for diagnosing a catalyst, comprising: adjusting a catalyst degradation threshold in response to a slope of a catalyst monitor sensor output; indicating catalyst degradation in response to a variable not exceeding the catalyst degradation threshold; and adjusting a state of an actuator in response to the indication of degradation.
By adjusting a catalyst diagnostic limit in response to an attribute of a catalyst monitor sensor, it may be possible to provide the technical result of improving catalyst diagnostics. In particular, an output voltage of a catalyst monitor sensor may change with age such that when the catalyst monitor sensor is part of a diagnostic to determine catalyst performance, catalyst oxygen storage may be overestimated. However, the catalyst oxygen storage estimate limit and a threshold amount of fuel provided to reactivate the catalyst may be adjusted to compensate for the catalyst sensor dynamics. Specifically, the oxygen storage estimate limit and the threshold fuel amount to reactivate the catalyst and provide rich exhaust constituent breakthrough downstream of the catalyst may be increased in response to a change in catalyst monitor sensor slope, or alternatively the catalyst monitor sensor time constant.
The present description may provide several advantages. For example, the approach may provide more reliable catalyst diagnostic evaluations. Further, the approach may reduce vehicle emissions via providing an indication of degraded catalyst operation. Further still, the approach may comply with on board diagnostic regulations.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.